This invention relates to the manufacture of bottles or containers of thermoplastic materials for the retention of fluids under pressure, such as carbonated beverages or the like. Recently, various thermoplastic materials have been developed which are capable of retaining carbon dioxide and which are blow-moldable into suitable containers. Such materials include poly(ethylene terephthalate) or PET, or nitrile based resins known as LOPAC, a registered trademark of Monsanto Company, or nitrile-group-containing monomers of the type disclosed in U.S. Pat. No. 3,873,660.
Such a bottle generally consists of a shoulder portion with a cap-receiving finish, a side wall or main body portion, and a bottom wall joined to the side wall. Pressure-retaining bottles are generally of cylindrical overall contour, but the present invention is applicable to bottles of other than cylindrical contour. For purposes of simplicity of description, such terms as "cylindrical", "annular", etc. are herein utilized, but it should be understood that these terms are merely descriptive, not limiting, in a geometric sense.
One primary problem which is encountered in blow-molding thermoplastic materials to form bottles capable of retaining CO.sub.2 and other gases under pressure resides in the provision of a bottom shape capable of serving as a bottle support, while resisting deformation under pressure.
One suitable bottom shape is a simple, outwardly hemispherical shape, but this requires a separately applied, outer peripheral support to make the bottle stand upright. A less expensive, more practical shape results from the inversion of the outwardly hemispherical shape to an inwardly concave or "champaign bottom" shape. The transition region located at the juncture of the cylindrical bottle side wall with the inverted, concave bottom forms a seating ring upon which the bottle is supported in an upright position. Much effort has been devoted to the design of inverted, concave bottoms of this type, and many different methods and many different molds have been developed.
To reduce the creep characteristics of polymeric materials under internal pressure, the material is oriented during the bottle formation, requiring blowing at a reduced temperature. Attempts to form a concave bottom by directly inflating a parison in a blow mold of the final bottle shape have failed. Under these blowing conditions, the material simply "bridges over" the sharp curvatures required in the mold to form an adequate seating ring, and the material stretches and thins out in the region where the greatest strength is required. As a result, seating rings deform under internal pressure to reduce the seating ring diameter and to change the pressure-resistant characteristic of the concave bottom.
It has been proposed that an initial outwardly convex bottom be blown which is then inverted to form a final concave bottom. Those methods and apparatus heretofore proposed either (1) require the utilization of a separate inversion mold and reheating of the initial bottom or (2) simply push a convex die against the outwardly convex bottom. Neither technique has solved the problems inherent in the requirements of sharp curvatures in the transition zone and of adequate material thickness at the seating ring.
As a result, the prior art has not yet evolved a method and apparatus for forming a concave, pressure-resistant bottom for a thermoplastic container capable of retaining fluids under pressure.